Structure of a Burkholderia pseudomallei Trimeric Autotransporter Adhesin Head

Pathogenic bacteria adhere to the host cell surface using a family of outer membrane proteins called Trimeric Autotransporter Adhesins (TAAs). Although TAAs are highly divergent in sequence and domain structure, they are all conceptually comprised of a C-terminal membrane anchoring domain and an N-terminal passenger domain. Passenger domains consist of a secretion sequence, a head region that facilitates binding to the host cell surface, and a stalk region.Pathogenic species of Burkholderia contain an overabundance of TAAs, some of which have been shown to elicit an immune response in the host. To understand the structural basis for host cell adhesion, we solved a 1.35 A resolution crystal structure of a BpaA TAA head domain from Burkholderia pseudomallei, the pathogen that causes melioidosis. The structure reveals a novel fold of an intricately intertwined trimer. The BpaA head is composed of structural elements that have been observed in other TAA head structures as well as several elements of previously unknown structure predicted from low sequence homology between TAAs. These elements are typically up to 40 amino acids long and are not domains, but rather modular structural elements that may be duplicated or omitted through evolution, creating molecular diversity among TAAs.The modular nature of BpaA, as demonstrated by its head domain crystal structure, and of TAAs in general provides insights into evolution of pathogen-host adhesion and may provide an avenue for diagnostics

Overall structure of <i>B. pseudomallei</i> BpaA third head domain.

<p>A. Monomeric structure of the third head domain of BpaA B. Quaternary trimeric structure of third head domain of BpaA C. Trimer top down view D. Bottom-up view.</p

Primary structure and domain annotation of BpaA from <i>Burkholderia pseudomallei</i>.

<p>A. Domain architecture of the <i>B. pseudomallei</i> BpaA trimeric autotransporter adhesin (TAA). The BpaA TAA features an N-terminal secretion sequence, four head domains, and a C-terminal membrane anchored domain. The residues between head domains are identified as regions of low complexity and the region between the fourth head domain and the membrane anchored domain is likely to be a coiled-coil. The third head domain contains numerous sequence motifs identified by the domain annotation of Trimeric Autotransporter Adhesins (daTAA) server <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0012803#pone.0012803-Szczesny1" target="_blank">[3]</a> B. Multiple sequence alignment of four head domains of <i>B. pseudomallei</i> BpaA The four head domains were aligned according to their sequence and motifs identified by the daTAA are indicated.</p

Sequence motifs in the third head domain of <i>B. pseudomallei</i> BpaA Trimeric Autotransporter Adhesin (TAA).

<p>Coloring indicates sequence motifs identified by the daTAA <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0012803#pone.0012803-Szczesny1" target="_blank">[3]</a> which is the same as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0012803#pone-0012803-g001" target="_blank">Figure 1</a>. A) BpaA third head domain monomer B) BpaA third head domain trimer C) FGG motif D) HANS motif. Hydrogen bonds shown as dashed lines.</p

Surface and interior structure of <i>B. pseudomallei</i> BpaA third head domain.

<p>A. One monomer of the BpaA head is sown in green ribbons with side chains shown in stick representation, while the other two monomers of the trimer are shown as a translucent molecular surface rendering in gray. B. Electrostatic surface potential mapped onto a molecular surface rendering of the third BpaA head domain. Blue indicates regions of positive charge and red indicates regions of negative charge.</p

Comparison of architecture of TAA head domain sequence motifs.

<p>Trimeric structures are shown for the TAA head domains of BadA from <i>Bartonella henselae</i> (PDB ID 3D9X <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0012803#pone.0012803-Szczesny2" target="_blank">[11]</a>), HiaBD2 (PDB ID 3EMF <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0012803#pone.0012803-Meng1" target="_blank">[9]</a>), KG1-W3 (PDB ID 3EMI <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0012803#pone.0012803-Meng1" target="_blank">[9]</a>), and HiaBD1 (PDB ID 1S7M <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0012803#pone.0012803-Yeo1" target="_blank">[10]</a>), from <i>Haemophilus influenzae</i>, YadA from <i>Yersinia enterocolitica</i> (PDB ID 1P9H <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0012803#pone.0012803-Nummelin1" target="_blank">[7]</a>), and BpaA from <i>B. pseudomallei</i> (PDB ID 3LAA). Trp-ring motifs are shown in yellow, GIN motifs are shown in light blue, neck regions are shown in brown, left handed β-roll Ylhead repeats are shown in magenta, FGG motifs are shown in dark blue, HANS motifs are shown in green, HIN2 motifs are shown in orange, and other regions in gray.</p